The present invention relates to a novel composition of matter which is a specific crystalline form of the pivaloyloxymethyl ester of the carbapenem derivative known as (1R,5S,6S)-2-[(4R)-2-oxo-4-pyrrolidinylthio]-6-[(1R)-1-hydroxyethyl]-1-met hyl -1-carbapen-2-em-3-carboxylic acid.
The compound has the formula: ##STR1## Carbapenem derivatives of this type have good activity as antibiotics and have potential as orally administrable pharmaceutical preparations.
U.S. Pat. No. 5,104,867 and European Patent Publication No. 337 637 disclose (in Example 39) the preparation of a carbapenem derivative which is related to the compound of formula (I) above. By following the prior art procedure of these patents, the carbapenem derivative has previously only been made in the form of an amorphous powder. However, our investigations have shown that the compound of formula (I) which is obtained as an amorphous powder, when prepared according to the methods described in the prior art, is difficult to formulate as an oral pharmaceutical preparation, as well as being slightly unstable, particularly after storage for long periods and is, therefore, of limited practical use as a drug.
We have now surprisingly found that a previously unrealised crystalline form of this compound shows a remarkable and altogether unexpected stability, which makes this carbapenem derivative much easier to handle and formulate and therefore of much greater value as a pharmaceutical.